In the formation of semiconductor devices, it is desirable to create spacers on the sidewalls of a gate to serve as masks during source/drain implantation. An exemplary spacer formation process and implantation process is depicted in FIGS. 1–3 described below.
In FIG. 1, a substrate 10 has a gate 12 formed thereon. The gate 12 may be a polysilicon gate, for example. Source/drain extension regions 16 are formed, by ion implantation, for example, employing the gate 12 as an implantation mask. A spacer layer 14 is deposited by any suitable method, such as chemical vapor deposition. The spacer layer may be made of any suitable material, such as silicon nitride, silicon oxide, low-k dielectric materials, etc.
An isotropic etching is performed, the results of which are depicted in FIG. 2. The isotropic etching creates “D”-shaped spacers 18 that extend from the sidewalls of gate 12. The spacers 18 form a mask, along with the gate 12, for performing a source/drain implantation process. It is desirable to space the deep source/drains from the gate 12 in order to reduce short channel effects. An ion implantation process, as indicated by arrows 20, is employed to create the deep source/drain implants. However, as will be recognized by the configuration of the D-shaped spacers 18 in FIG. 2, the outer regions of the D-shaped spacers 18 have a relatively thin profile at the outer edges. This allows some “punch-through” of the ions during the deep source/drain implantation process, which is a relatively high-energy process.
The consequences of the punch-through due to the thin profile of the spacers 18 is depicted in FIG. 3. An impressive control of the location of the deep source/drain implants creates the formation of source/drain regions 22 that extend underneath the spacers 18 to the area indicated by 24. This is further than the desirable region indicated by 26 in FIG. 3. Hence, the channel has been undesirably shortened due to the punch-through of the D-shaped spacers 18 during the source/drain implantation process.
Another disadvantage of the D-shaped spacers 18 is the worsening of conformity for further film deposition, necessary in such processes as the formation of a double spacer or the forming of an interconnect layer dielectric. The sloping nature of the outer shape of the spacers reduces conformity in subsequent film depositions.